The market and the need to develop efficient portable electronic equipment have pushed the industry to produce circuit designs with very low voltage (LV) power supply and also often constrained to low power (LP) consumption. The basic problem with using conventional analog differential and/or operational amplifiers in fine-line CMOS technology is that the threshold voltage and drain source saturation voltage do not scale down at the same rate as the supply voltage.
An operational amplifier, the most omnipresent analog system building block, has had to adapt in order to function in today's low voltage, high noise environment. Therefore, fully differential design principles have been applied to the operational amplifiers.
However, when a differential amplifier is in a feedback configuration, a high differential gain of a fully differential amplifier stabilizes differential-mode signals within the amplifier, but common-mode signals can float. Extra circuitry, called a common mode feedback (CMFB) circuit, is required to increase a common mode loop gain of the amplifier so that the common-mode signals are stabilized. The CMFB circuit implements a negative feedback loop that must be compensated properly to minimize loop settling time and to maintain stability. Often, designing the CMFB circuit is more challenging than the actual operational amplifier's design due to the difficulty in properly compensating the CMFB circuit.
The CMFB circuit averages both differential output voltages to produce a common mode voltage VCM. The voltage VCM is then compared to a desired reference common-mode voltage VCMR. A difference between VCM and VCMR is amplified and this error voltage is used to change the common mode bias current. If the common-mode voltage VCM is continuously compared with a constant reference voltage VCMR, then the common mode feedback circuit is referred as a continuous time common mode feedback circuit.
FIG. 1 illustrates a conventional continuous time common mode feedback circuit 100. The circuit 100 includes four identical transistors 102, 104, 106, 108 and two transistors 110 and 112. The common mode feedback circuit 100 averages two differential signals VCMP and VCMM that are supplied by differential outputs OUTP and OUTM of a main differential amplifier (not shown) and compares the average to the reference common mode voltage VCMR by using the four identical transistors 102, 104, 106 and 108 configured into two differential pairs.
Currents through the transistors 102, 104, 106 and 108 are given by the following equations:IDP0=IB/2−ΔI; IDP1=IB/2+ΔI; IDP2=IB/2−ΔI; IDP3=IB/2+ΔI; 
Where IDP0 is the current through the transistor 102, IDP1 is the current through the transistor 104, IDP2 is the current through the transistor 106, IDP3 is the current through the transistor 108 and IB is the current flowing to the transistors 102, 104, 106 and 108.
The current IDP0 equals IDP2 and IDP1 equals IDP3. Currents through the transistors 110 and 112 are equal to IB. Now if the differential signals VCMP and VCMM are averaged with VCM which is greater than VCMR, currents through the transistors 102 and 108, IDP0 and IDP3 will decrease causing currents IDP1 and IDP2 through transistors 104 and 106 to increase. The increased current in the diode connected transistor 110 causes a voltage VCNTRL to increase. The increase in the voltage VCNTRL is applied to the gate of the transistors which are part of the operational amplifier output stage current sink (not shown). The current being sunk into the drain of those transistors will increase, causing a reduction in the voltage of nodes VCMP and VCMM, thus reducing the common mode output voltage VCM. The same analysis can be used to describe the behavior of the CMFB circuit when the differential signals VCMP and VCMM are averaged with VCM smaller than VCMR.
However, this conventional method cannot be used in low voltage power supply environment without severely limiting an output voltage swing (VCMP-VCMM) of the operational amplifier and the non-linear behavior of the differential pairs of the circuit 100.
Some other conventional common mode feedback schemes are also used. In one method, the common mode voltage VCM is periodically refreshed to the common mode reference voltage VCMR. This circuit is well suited for low voltage applications to increase the output voltage swing. However, as the circuit is based on switched capacitor technique, additional switching noise is introduced to output signals.
Another conventional common mode feedback scheme is designed with resistors. The amplifiers have problems of resistor tolerances and the large value resistors degrade the performance of the amplifiers by limiting the voltage swing, thereby forcing operation at higher supply voltages where the limited voltage swing is not a disadvantage.
Therefore, there is a need of a novel continuous time common mode feedback circuit for low voltage operational amplifiers for providing a good linearity, a wide bandwidth and a low systematic offset.